fix(instructions): prevent special parameter name collisions

[ss-o]

Summary

• add a required Zsh rule that prevents unrelated local or scratch data from
  reusing shell-owned special parameter names
• document the concrete status and path failure modes and recommend
  purpose-specific names
• synchronize policy metadata, validator inventories, startup membership, and
  deterministic regression coverage

Closes #546

Instruction impact review

1. Classification: scoped normative Zsh guidance plus existing
   machine-readable enforcement metadata.
2. Consumers and contexts: Codex, Claude Code, Copilot, Gemini CLI, and
   human contributors working with enrolled Zsh source in all five execution
   profiles.
3. Canonical owner: .github/instructions/zsh-scripting.instructions.md
   remains the prose owner; lib/zsh-standard-policy.json remains the metadata
   owner.
4. Duplication or contradiction: none found. Existing rules cover explicit
   scope and dynamic scope, not collisions with special parameter names.
5. Manifest routing: unchanged. The existing instruction-zsh-scripting
   route already reaches the affected paths and consumers.
6. Runtime delivery: all supported runtimes receive the mandatory scoped
   instruction without relying on an optional hook or skill.
7. Generated output and limits: the public policy validators pass, and the
   private generated root composite is unaffected because neither imported
   baseline policy nor the private overlay changes.

Verification

• python3 -m unittest scripts/test_validate_zsh_standard_policy.py -v
  (99 tests passed)
• python3 -m unittest scripts/test_validate_agent_policy.py -v
  (83 tests passed)
• python3 scripts/validate-zsh-standard-policy.py
• python3 scripts/validate-agent-policy.py
• git diff --check

The rule semantics were checked against the released Zsh Parameters manual and
confirmed with isolated zsh -f probes.